In certain marking engines (e.g. copier, duplicator, printer, etc.), a continuous loop of a photoconductor film is commonly used to transfer an image from an input section onto a marking medium (e.g. a sheet of paper or the like). The film is initially charged and passed through the input section where an image is projected onto the charged film. The film then moves through a developing section where toner is applied to the charged image, and on through an image transfer section where the toner image is transferred onto a sheet of paper or some other marking medium. The toner image is then fixed (i.e. fused) to the sheet by passing the sheet between a pressure roller and a heated roller within the fuser section of the machine.
In such machines, it is common to use a vacuum transport to transfer the sheet from the image transfer section (i.e. film loop) to the fuser section. Often this vacuum transport is directly interfaced between the film and the fuser section whereby the vacuum transport receives the sheet from the film and passes it directly into nip between the rollers in the fuser section. This requires that the surface speeds of (a) the film loop, (b) the vacuum transport belt(s), and (c) the fuser rollers all have to be closely matched. If the speeds become mismatched, there may be relative movement between the film and the sheet while the image is being transferred thereby resulting in smearing of the image on the sheet.
To alleviate this problem, some commercial machines have abandoned such a direct interface and instead, now use a extended travel path between the image transfer and fuser section which is longer than the straight-line distance between these two sections (i.e. longer than the length of any sheet to be used in the marking operations). This extended path effectively “de-couples” the speed of the fuser rollers from the speed of the film thereby eliminating the possibility of relative movement between the sheet and the film as the toner image is being transferred onto the sheet.
The travel path is extended by angling the vacuum transport away from the straight-line distance between the sections and then positioning a fuser entrance guide between the exit end of the vacuum transport and the entrance of the fuser section. The fuser guide is normally vacuum assisted which holds the sheet against the surface of the guide as the sheet moves from the vacuum transport into the fuser section. This type of extended travel path and vacuum-assisted guide is known and has been used in commercially available machines, e.g. DIGIMASTER 9110, Heidelberg Digital L.L.C., Rochester, N.Y.
As a sheet moves along the extended travel path, it is particularly important to prevent the sheet (e.g. trail edge of sheet) from falling away from the fuser entrance guide as it moves across the surface of the guide. If the sheet should sag or drop, it may contact and slide across other elements in the paper path before it enters the fuser section which, in turn, is likely to cause smearing of the unfused image on the sheet thereby making the print unacceptable to the user.
To prevent such sagging, the vacuum being applied at the guide must be strong enough to hold even the heaviest sheet used in marking operations firmly in contact with the guide's surface until the trail edge of the sheet has completely entered the nip between the fuser rollers. Unfortunately, this may be difficult to achieve since a vacuum that is strong enough to hold the heavier sheets against the guide's surface may also be too strong to allow those sheets to readily move across the surface. That is, if the vacuum is too strong, it may adversely affect the ability of the vacuum transport to move the sheet across the guide. When this happens, the sheet slows or stalls completely on the guide's surface thereby resulting in serious jamming problems or the like.
Therefore, it is important to provide a vacuum force on the guide which will hold even the heaviest sheet in contact with guide surface until the sheet has moved completely across the surface of the guide but, at the same time, will allow the vacuum transport to readily move the lead edge of the sheet across the guide and into the nip between the fuser rollers without the sheet slowing or stalling on the guide's surface.